This invention relates to the reinforcement of castable materials, particularly but not exclusively mortar or concrete, by the distribution therein at the mixing stage of a multiplicity of comparatively small elongate metallic elements, e.g. of steel or iron. Such elements may each comprise a single metal filament or they may comprise two or more such filaments combined together, e.g. in the form of a twisted strand, either of which possibilities is intended to be within the scope of the term "reinforcing element" as used in this specification. More specific examples of such elements are ones made of round wire, ones made of straight or helically twisted strip, or made with lengthwise varying cross-section to improve the adherence with the castable material, and ones made of two, three, or preferably four round wires (e.g. each of 0.175 mm diameter) twisted together in the form of a cable. The thickness of such small metallic elements generally ranges between about 0.1 mm and 1 mm, and the length-to-thickness ratio usually ranges between about 50 and 200.
In the field of mortar and concrete it is known that the tensile strength of the material rises approximately linearly with increasing percentage of such reinforcing elements which are equally distributed throughout the material, and this principle can also be used in other castable materials, in particular those which use a water-activated binder. These are materials comprising a binder such as chalk or cement, which hardens by mixing with water and binds together the other granular ingredients, such as sand, of the material. Such materials are usually formed by mixing together the necessary ingredients, such as cement, and/or other water-activated binder, sand, aggragate, additional polymer material in some cases, and water, into a soft mix ready to harden as the binder is activated. The reinforcing elements can be added before or during mixing of the ingredients together. To obtain high strength, it is desirable to introduce a high percentage of reinforcing elements into the mixture, but there is a limit to this percentage due to the reinforcing elements entangling with each other to form balls and other undesired conglomerations.
The length-to-thickness ratio of the reinforcing elements is chosen as a compromise between the requirement of good mixability of the reinforcing elements and the requirement of good reinforcing effect per volume percent of reinforcing elements. On one hand, for good mixability short thick elements are to be preferred. But on the other hand such elements are undesirably thick and strong from the point of view of adherence to the surrounding material, as a result of which the amount of metal used is not distributed so as to ensure an optimal reinforcing effect. Usually, and especially in the field of mortar and concrete, a length-to-thickness ratio is chosen in a range between 50 and 200, preferably between 70 to 160. For non-straight elements the length is measured not as the developed length of the element after it has been straightened, but as the rectilinear distance from extremity to extremity. For elements of non-circular or non-constant cross-section the thickness is measured as the diameter (or the average diameter over the different cross-sections if appropriate) of a circle of the same cross-sectional area. The thickness of the elements used generally ranges between 0.1 mm and 1 mm.
But even within this compromise range of length-to-thickness ratio it is difficult to attain desirably high strength of the reinforced material, because good mixability and good reinforcing effect of the reinforcing elements are still both required but up to now cannot together be sufficiently achieved because the two requirements have been incompatible. In order to make the material as strong as possible it has been necessary up to now to introduce and mix the elements in percentages approaching the limit of mixability and taking the utmost care to prevent entanglement. The reinforcing elements are however delivered in containers in which they are already entangled with each other due to vibrations and shocks during transport. This mass of entangled elements does not lend itself to pouring a desired dose into the mixture. As a second drawback, the reinforcing elements of this entangled mass cannot be poured into the mix in percentages approaching the limit of mixability, because at these percentages the mixing movement will not cause disentanglement.
A known way of reducing entanglement when introducing a high percentage of reinforcing elements into a mix is to introduce them in the form of a continuous rain of separate elements. The mass of reinforcing elements is poured into a hopper located above the mix, and the bottom exit for the reinforcing elements comprises a mill where the elements are separated from each other by mechanical or pneumatic means and are separately dropped into the mix. This solution is not very practical because it requires additional apparatus which is an additional cost, takes up room and makes the mix less accessible. Furthermore, it requires an uneconomically long time for introducing the reinforcing elements into the mix.
Another method of reducing entanglement during mixing has been proposed in U.S. Pat. No. 3,716,386, where the fibers are firstly treated with a high-viscosity friction-reducing substance prior to the bringing together of the fiber constituent with the basic constituent of the mix. This method makes the mixing operation more complicated and does not allow to prevent entanglement in the containers during transport.
As a result, although the technique of using reinforcing elements of the kind described to reinforce mortar or concrete can yield a high strength product, the problem of how to deliver the reinforcing elements and how to mix them in a simple way so as to prevent entanglement, is still a factor causing reluctance to adopt this technique on a large scale, and also prevents the achievement of strength values with a minimum amount of steel which could make this technique particularly advantageous from the point of view of competition with conventional reinforcement.
According to the invention, the process of introducing the small elongate metallic reinforcing elements into the castable material comprises introducing into a mix for said material a multiplicity of reinforcing members being in the form of a group of such elements combined together by a binder affectable by a disintegration ingredient, then mixing said mix to distribute said reinforcing members substantially uniformly therein, then causing said reinforcing members to subdivide and further to disintegrate into separate elements by means of said disintegration ingredient, and then further mixing said mix to distribute said separate elements uniformly therein.
Viewed from another aspect, the invention provides the reinforcing members being in the form of comparatively small elongate metallic reinforcing elements such like steel or iron fibers, combined together in a group by a binder affectable by a disintegration ingredient suitable to be included in the castable material.
The invention is particularly applicable in the field of castable material on the basis of water-activated binder, such like mortar and concrete, as herein described. The preferred range of thickness (or diameter in the case of a circular cross-section) lies then between 0.1 mm and 1 mm and preferred lenght-to-thickness ratio (or length-to-diameter ratio) ranges then between 50 and 200, more preferably between 70 and 160. Although these limits are not to be considered as absolute limits, the elements between this limits give a greatly superior combination of results to elements substantially outside these limits.
In short, the invention provides the method of mixing the reinforcing elements in two periods: a first period of uniformly mixing, from a macroscopical point of view, the elements which are kept in the form of small groups of combined reinforcing elements, which groups are uniformly distributed, and a second period of uniformly mixing the individual elements, from a microscopical point of view, after the groups have disintegrated. The danger of entanglement during the first period is low because the combined groups have better mixability than the individual elements, and the danger during the second period is also low because this second period can be kept short, as a result of the preliminary macroscopic distribution.
The reinforcing elements are preferably made of hard drawn steel having a tensile strength of at least 85 kg/mm.sup.2, preferably at least 120 kg/mm.sup.2 but they may be made of cast iron as disclosed e.g. in French Pat. No. 2,091,734. Iron alloyed with other metals, such as nickel or chrome for improving corrosion resistance, is also possible. A zinc, aluminum, organic or other coating can also be applied, for improving adhesion and/or corrosion resistance.
As already mentioned individual reinforcing elements easily form into entangled conglomerations which have a strong resistance to disintegration. In carrying out the invention, the reinforcing elements are purposely provided in the form of small groups of such elements combined together by a binder, and so forming the reinforcing members, but taking care of two points.
Firstly, the reinforcing members shall have a higher mixability than the individual reinforcing elements. This means that a group structure must be chosen whose tendency to entanglement into greater conglomerations is smaller than the same tendency of the individual reinforcing elements. It will be clear for those skilled in the art what group structures should be chosen and what should not, but instructions for a good choice will be given below. The mixability, or the inverse of the tendency to entanglement is measured by the maximum percentage which can be introduced, in the form of a continuous rain, into a mix before conglomeration into entangled balls occurs.
Secondly, the binder used in the reinforcing member must fulfill certain conditions. On one hand, the reinforcing members must have the time to be mixed until substantially uniform distribution in the mix, before the occurrence of any substantial subdivision as the start for further disintegration into separate reinforcing elements. This needs at any rate a sufficient bonding strength of the binder, so as to keep the reinforcing elements together and to allow the reinforcing members to endure the mixing movement during that time without substantial subdivision. On the other hand the binder must be sufficiently affectable by another ingredient of the mix, in order to allow the subdivision and further disintegration of the reinforcing members at the desired moment. This is achieved by using a binder which is, e.g. soluble in water. Such other ingredient may be regarded as the disintegration ingredient.
The desired moment of subdivision and further disintegration can then be controlled either by choice of the moment of introduction of the said disintegration ingredient or by the choice of an appropriate duration of resistance of the binder to the disintegration ingredient. (The latter effect may be obtained, for example, by varying the thickness of the binder film or varying the composition of soluble and insoluble materials in the binder.) In the first case the disintegration ingredient is introduced in the mix after introduction of said reinforcing members, and the mixing to distribute said reinforcing members is carried through during the delay time between introduction of the reinforcing members and the moment when said members are substantially subdivided. In the second case said reinforcing members are introduced in the mix comprising said disintegration ingredient (because it is introduced at the same time or was already introduced before), and the mixing to distribute said reinforcing members is carried through during the delay time before said members are substantially subdivided. In this second case, reinforcing members are chosen having a resistance to the disintegration ingredient which procures said delay time.
Thus during the first mixing period any subdivision of the reinforcing members or of the groups of reinforcing elements is retarded by an appropriate control of the said delay time between introduction and substantial subdivision of the reinforcing members. During that time the groups are well mixed before substantial subdivision occurs. Substantial subdivision of the groups may be regarded as having been reached when the total remaining number of whole groups and part groups is treble the intial number of groups introduced. If an unreasonable number of groups is not introduced there is no danger of entanglement into balls during the first mixing period, because the groups can be formed to have less tendency to entangle then the individual reinforcing elements.
The second mixing period can be regarded as starting when substantial subdivision is reached. Then the groups rapidly continue to subdivide until substantial disintegration into separate reinforcing elements is reached and these elements must then rapidly be mixed because the danger of entanglement into balls exists. But the second period can be kept short, thanks to the substantially equal distribution of the groups obtained during the first period, in which substantially no danger of such entanglement existed. If not introduced in unreasonable numbers, reinforcing elements in sufficient concentration to cause such entanglement in the long run are not given the time to entangle before the mix is ready. So it has been observed that this mixing in two stages leads to higher percentages of reinforcing elements being mixable without entanglement into balls than if they has been introduced as separate elements.
By being brought together in groups, and forming reinforcement members, the mass of reinforcing elements shows a lower tendency to entangle during transport and during pouring out into the mix, and this allows in most cases the mass to be dumped in bulk into the mix instead of forming a continuous rain.
It is to be clearly understood that the process of the invention should not be regarded as being divided into separate and distinct stages in a first one of which the groups of reinforcing elements are distributed and in a second of which they subdivide and disintegrate into separate reinforcing elements and in a third of which the separated elements are mixed, because more often than not there will be some overlap of these stages. This is particularly so, of course, when the disintegration ingredient is present in the mix when the groups of reinforcing elements are first introduced, as a greater or less degree of subdivision of individual groups is then almost bound to occur during the first mixing stage.
Accordingly, a primary object of this invention is to provide reinforcing members for settable material which overcome the disadvantages of prior art reinforcements.
Another object of this invention is to provide reinforcing members formed of discrete reinforcing elements which members dissociate during mixing with a settable material.
A further object of this invention is to provide reinforcing members formed of discrete reinforcing elements united by a binder which is affected by one or more ingredients in the settable material so as to release the elements.
Still another object of this invention is to provide reinforcing members for a settable material which enable more random and uniform dispersal of reinforcement throughout the material.
Still a further object of this invention is to provide reinforcing members for a settable material which enable the introduction of a greater percentage of reinforcing elements into the material without balling up of the elements.
Yet another object of this invention is to provide an improved method for the introduction of reinforcing wires into a hardenable material.
Yet another object of this invention is to provide a method for introducing a greater percentage of reinforcing elements into a settable material randomly.
Another object of this invention is to provide an improved method for reinforcing a settable material.
A further object of this invention is to provide a method for producing improved reinforcing members.